Position location technologies typically utilize wireless signals concurrently transmitted from known locations to determine position. In many prior art technologies, the wireless signals are concurrently transmitted from a multiplicity of satellites at a known time, and with a predefined frequency. On the ground, a satellite receiver acquires a signal from each satellite within its view of the sky. The times of arrival of the signals along with the exact location of the in-view satellites and the exact times the signals were transmitted from each satellite are used to locate the position of the satellite receiver, via a trilateration calculation well known in the art.
There are two principal functions implemented by an exemplary prior art receiver of such satellite signals: (1) computation of the pseudoranges to the various satellites, and (2) based on these pseudoranges, computation of the position of the satellite receiver, satellite timing, and ephemeris (position) data. Pseudoranges (PRs) measure the time delays (or equivalently the ranges) between the satellites and the receiver, with a bias due to (a) the local clock in the receiver and (b) satellite clock. In conventional autonomous satellite receivers, the satellite ephemeris and time of transmission data are extracted from the satellite signal, once the satellite signal is acquired and tracked. Collecting this information in a prior art receiver can take a relatively long time (e.g. 30 seconds). Alternatively, this information may be received as a part of GPS aiding information from a server and therefore could take less time.
Specifically, after a wireless signal has been received from a satellite and filtered, followed by down conversion to baseband, and correlation of the resulting signal 10 (FIG. 1) with a reference signal. The correlation typically shifts (e.g. 1023 chips of) the two signals relative to one another, followed by multiplication, followed by summing (in one example 1023) products resulting from multiplication. For example, a satellite receiver of the prior art multiplies signal 10 (in baseband) by a reference signal (not shown) that is locally generated to contain a stored replica of a predetermined code retrieved from its local memory. The results of multiplication (also called “correlation”) over several milliseconds are then added up (i.e. “integrated”), to extract binary data in signal 10. The just-described predetermined code may be, for example, a Pseudorandom Noise (PN) code, such as a coarse/acquisition (C/A) code generated at a rate of 1.023 MHz that repeats every millisecond, unique to each Global Positioning System (GPS) satellite. Note that in some prior art GPS receivers, coherent integration over periods that exceed one data bit requires the receiver to have knowledge of the bit's value and location in signal 10, relative to a local clock in the receiver.
Some prior art receivers process signal 10 sequentially in three stages as follows. A first stage 15 (see FIG. 1) performs for example down-converting to baseband, signal acquisition (e.g. correlation, integration and peak processing to identify which satellites are present, to identify the code phase within each satellite's signal and to identify the frequency of each satellite's signal), and verification are performed in time period δT0 shown in FIG. 1. Such a receiver then detects the occurrence of an edge of a bit of digital data from a specific satellite carried by signal 10 (e.g. during time period δT1 shown in FIG. 1, typically 1 second), in a second stage 16 called “bit edge detection”. After a bit edge is detected, the bit edge's location in signal 10 is used in a third stage 17, to correlate the specific satellite's signal with the reference signal corresponding to the specific satellite that was acquired, aligned to the edge of the data bit, and to integrate (i.e. add up) results of correlation, to generate sums (“integration sums”). Bit edge detection has other uses, e.g. the bit edge is also used in pseudorange (PR) computation.
After successful completion of bit edge detection, a fourth stage 18 called “data demodulation” begins, wherein the receiver determines the values of data bits in signal 10, based on the integration sums. By the time the fourth stage 18 begins, a predetermined number of integration sums are generated between time δT0+δT1 and time δT0+δT1+δT2 as shown in FIG. 1. Generation of data bits from integration sums is well known in the art of data demodulation of a GPS signal, and not described further herein. Note that in such a prior art method, the period of time required to start data demodulating is δT0+δT1+δT2. Demodulation stage 18 is performed in several substages, 18A, 18B etc. because the time δT3 required for demodulation of integration sums is less than the time δT2 required for correlation and integration in stage 17.
While performing data demodulation in first substage 18A (FIG. 1), the prior art receiver continues to perform correlation and continues to generate integration sums in stage 17. Specifically, between time δT0+δT1+δT2 and time δT0+δT1+2δT2, additional results of correlation are integrated and additional normally-collected sums are stored. Therefore, after time δT0+δT1+2δT2, the just-described data demodulation is repeated in a second substage 18B (FIG. 1) using the additional normally-collected sums, thereby to generate additional data bits from signal 10. Accordingly, correlation, integration of a number of results of correlating, and data demodulation may be repeated any number of times, to generate any number of data bits from signal 10. If each data bit is 20 ms in duration, then fifty data bits are generated (i.e. extracted) by each substage in demodulation stage 18. The data bits extracted from signal 10 are eventually used to determine navigation data. The navigation data includes all data transmitted by the satellite, for example, satellite position, satellite clock, information about other satellites (e.g. almanac), etc. Such navigation data may be used to compute the position of the receiver, timing at the transmitter, ephemeris (position) data, and satellite clock correction data.
For more information on signal acquisition, downconverting to baseband, correlating and verification see U.S. Pat. Nos. 6,313,786, 6,185,427, 6,928,275 and US Publication 2006-0114984 all of which are incorporated by reference herein in their entirety. For background information on detection of an edge of a data bit (also called “bit edge transition” or “bit edge boundary”) see US Publication 20090219202 which is also incorporated by reference herein in its entirety. For background information on demodulation to generate data bits, see US Publications 20090215419, 20080049857 and 20020031192 all of which are incorporated by reference herein in their entirety.
The inventors of this current patent application believe it would be beneficial to reduce the prior art duration required to start data demodulation, to a time period that is less than δT0+δT1+δT2 (see FIG. 1). For example, the inventors believe that an early start of data demodulation allows the location of a cellular phone to be determined quickly, in case an emergency number is being dialed.